Our functional E-neoantigen (Eneo) radioimmunoassay (RIA) determines normal and pathological plasma levels of fibrogen-fibrin degradation products (FDP). This RIA measures Eneo immunoreactivities of fragments X, Y, and E engendered by the plasmin cleavage of fibrinogen (Fg) and fibrin (Fb). Although the RIA cannot discriminate immunochemically between Fg-E and Fb-E, it does give a parallel relationship between logit transforms of the inhibition curves of test plasmas and Fg-E standard in competition with the reaction between 125I-Fg-E and Fg-absorbed anti-Fg-E serum. Quantitative recovery of Fg-E added to test plasma also validates the assay. Plasma Eneo immunoreactivity levels have been determined in 150 cancer patients. Normal values ranged 4-22 ng/ml; patients with multiple forms of neoplasms were normal-277 ng/ml. Eneo levels were considerably higher in patients with consumption coagulopathy. The molecular weight of Eneo immunoreactivity in pathological plasmas also was determined. Initial efforts are underway to establish a solid phase RIA by utilizing essentially two different approaches. In vitro generation of Fg-Eneo expression is being examined in two systems: (1) purified Fg digested by urokinase-activated plasminogen, and (2) normal plasma degraded by plasmin. The Eneo immunoreactivity of a series of timed Fg digests will be determined by RIA.